The ministry was now in the process of setting up a national case investigation service, which was expected to increase capacity by a further 150 cases each day, while also allowing it to be ramped up over time.
While the system public health units were operating under had been designed to manage between 180 and 270 cases a day during the national lockdown, tracing efforts during Auckland's delta outbreak fell short of those totals.
The ministry conceded "very high levels of complexity" seen in cases during October had "impacted the estimations" of what could be achieved under a system that also followed up on casual contacts.
"Taking that approach, at that time, meant that it was estimated that PHUs had capacity to manage between 100 to 120 cases per day, with the ability to surge up to 150 cases per day for limited periods of time."
With case numbers on a rising trajectory – health experts warned this week the daily tally could hit 1000 by Christmas if the spread escalates further – Covid-19 modeller Professor Shaun Hendy said a change in approach was necessary.
The new dynamic was illustrated by the fact locations of interest were no longer being widely shared, while far fewer of the cases disclosed each day were epidemiologically linked.
"The contingency plans that we did have in place were based on scenarios like August last year, where we could still eliminate, and we could stand up surge capacity to deal with a large number of cases for a short period of time," said Hendy, of Te Pūnaha Matatini.
"Whereas now we're moving into a different phase, where we've obviously got a tolerance for cases – particularly in Auckland – so what you need to have is a much more sustainable contact tracing approach."
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He expected that, while contact tracers would still be spending a lot of time reaching people in vulnerable parts in the community, our response would increasingly come to rely on electronic tools.
"Now that the outbreak is much more widely distributed across Auckland - and we're going to be moving into a traffic light system where we'll be tolerating cases around the country – we really do need scalable solutions."
That meant a regime that didn't require as much direct interaction with contact tracers, but relied more on peoples' ability to self-manage and self-isolate, he said.
Even so, he said contact tracing would continue to be critical, "because we know that vaccination is not enough to control delta alone".
"So, I think it really is important that we get a sustainable contact tracing system in place, that can run at reasonably high capacity for many months - even years."
University of Auckland technology expert Dr Andrew Chen agreed that more automation through tools like the tracer app could help boost caseload capacity.
While peoples' use of the app had been poor over time, he said recent data was more encouraging – particularly in Auckland.
"In level 3 step 2, for instance, we're seeing a 12 per cent increase in QR code scans each day relative to the same day last week, which I suspect is reflective of the increased mobility around Auckland," he said.
"We're now seeing numbers in the order of 2.7 million QR code daily scans. I would expect that, once everything is open, and everyone is out and about as they would be under level 1 conditions, then we might be seeing four to five million QR scans a day."
As for the other key part of the app, around 2.26 million people had switched on Bluetooth tracing – equivalent to around 60 per cent of the adult population.
"That means we've got a really high proportion of people using it relative to other jurisdictions – places like Europe, for instance, are quite happy to have 20 to 25 per cent of the population using Bluetooth contact tracing tools.
"Because we're at 60 per cent, we definitely should be thinking about how we can use that to support contact tracing processes."
